[tests] Test sites for identity in TestSiteObject.test_constructors
[tests] Test sites for identity in TestSiteObject.test_constructors

- sites created by site.APISite.fromDBName() must be identical to sites
  created by Site function because both of them uses pywikibot._sites
  object cache
- use sitename for instantiating a site

Change-Id: Id613a6fe921cd5af3cdcdb6d81cb1051eab12ee1

wikimedia/pywikibot-core,wikimedia/pywikibot-core